This invention relates to an apparatus for maintaining a braking force applied to a vehicle and, more particularly, to such apparatus which is also capable of starting a vehicle smoothly when the vehicle is at rest on an upgrade.
When a vehicle on an upgrade is to be moved forward, the driver of the conventional vehicle engages the hand brake with one hand to apply a braking force to the vehicle. Then, while engaging the clutch, the driver steps down on the accelerator pedal and slowly releases the hand brake to move the vehicle forward. In a case where the vehicle cannot be stopped on the upgrade effectively because the braking force applied by the hand brake alone is insufficient, the driver steps down on the brake pedal with his right foot to stop the vehicle, steps down on the clutch pedal with his left foot and puts the vehicle in gear. The driver then withdraws his left foot from the clutch pedal and transfers his right foot from the brake pedal to the accelerator pedal to raise the engine rpm. As the engine rpm is being raised, the vehicle is propelled forward when the clutch is engaged in concurrence with removal of the driver's foot from the brake pedal.
The coordinated operation of the hand brake, clutch pedal and accelerator pedal, or the operation of the clutch pedal in coordination with the changeover from the brake pedal to the accelerator pedal must be performed with certain timing or else the engine will stall or the vehicle will move backward. These operations require that the driver have considerable skill. A major accident can occur if the engine should happen to stall when attempting to start the stationary vehicle on an upgrade or at a railrod crossing. Hence there is a need for development of an apparatus for maintaining a braking force applied to a vehicle, which apparatus is also capable of starting a stopped vehicle smoothly on an upgrade or at a railroad crossing.
An apparatus of this type has already been proposed and is shown in FIG. 1. The apparatus has an electromagnetic check valve 3 disposed in a conduit interconnecting a master cylinder 1 and a brake oil cylinder 2 of a hydraulic brake unit. The electromagnetic check valve 3 is adapted for electromagnetic switching and includes a solenoid 4, a return spring 5 and a magnetically attractive core consisting of a magnetic body. The solenoid 4 constitutes part of an electrical circuit in which there are serially connected a switch 6 for sensing engagement of a clutch, and a switch 7 actuated in association with a device for sensing selection of a specific gear position, with relation to part of a gear position selection mechanism in a transmission. The switch 7 is connected to a storage battery 8.
Reference will now be had to FIGS. 2(a) and (b) to describe in detail the operatin of the braking force maintaining apparatus having the construction shown in FIG. 1.
FIGS. 2(a) and (b) illustrate an ordinary braking operation, namely a state in which the solenoid 4 of the electromagnetic valve 3 shown in FIG. 1 is deenergized. With the solenoid 4 deenergized, operating the master cylinder 1 by pressing down on the brake pedal 10 supplies hydraulic pressure to the brake oil cylinder 2 through hydraulic pressure routes 3a, 3b to apply the brake, as shown in FIG. 2(a). When the driver removes his foot from the brake pedal 10 with the solenoid 4 in the deenergized state, the hydraulic pressure is returned to the master cylinder 1 solely through the hydraulic route 3b, thereby releasing the brake, as shown in FIG. 2(b).
Next, with the solenoid 4 of the electromagnetic valve 3 in the energized state, the brake oil from the master cylinder 1, which is coupled to the brake pedal 10, flows into the brake oil cylinder 2 through the other oil route 3a to supply the brake oil cylinder 2 with hydraulic pressure and, hence, engage the brake. Under these conditions, the hydraulic routes 3a and 3b are both closed in the direction leading from the brake oil cylinder 2 to the master cylinder 1 to prevent backward flow of the brake oil from the brake oil cylinder 2. In other words, the braking pressure is maintained so that the brake will be held in the engaged state even when the brake pedal 10 is released. When the solenoid 4 is deenergized, however, the electromagnetic valve 3 opens the route 3b, whereby the brake brake oil maintained in the oil cylinder 2 is returned to the master cylinder 1 to disengage the brake.
Since this prior-art apparatus for maintaining braking force controls the electromagnetic check valve 3 by simple electrical circuitry which includes the switch 6 for sensing clutch engagement and the switch 7 for sensing the specific gear position of the transmission, it is difficult to control the electromagnetic check valve at the same time it is desired to move the vehicle to move from rest. As a result, smooth control of the braking force when starting the vehicle from rest cannot be achieved. For this reason, the prior-art apparatus described above has not yet been put into practical use.